Chromium carbide-nickel base alloys are known in the art as coatings to combat high static coefficients of friction and high wear rates of 316 stainless steel components in the core of sodium cooled reactors. The coatings for such application have to withstand high neutron irradiation, be resistant to liquid sodium, have thermal shock resistance and have good self-mating characteristics in terms of coefficient of friction and low wear rates. The published article titled "Sodium Compatibility Studies of Low Friction Carbide Coatings for Reactor Application", Paper No. 17, by G. A. Whitlow et al, Corrosion/74, Chicago, Ill., Mar. 4-8, 1974 discusses the effects of thermal cycling, compatibility with sodium, etc. on a variety of coatings including the detonation gun Cr.sub.3 C.sub.2 +Inconel 718 coating. Inconel is a trademark of International Nickel Company for nickel alloys. Testing included thermal cycling between 800.degree. F. and 1160.degree. F. for 1000 hours. After such exposure, there was no spalling or other mechanical damage to the Cr.sub.3 C.sub.2 +Inconel 718 coating, and there was no observable microstructural change using metallography other than changes within the substrate. X-ray evaluation of the microstructures, however, showed that the as-deposited coating contained Cr.sub.7 C.sub.3 plus Cr.sub.23 C.sub.6, and that there appeared to be a conversion of Cr.sub.7 C.sub.3 to Cr.sub.23 C.sub.6 on long term exposure at elevated temperatures. The detonation gun Cr.sub.3 C.sub.2 +Inconel 718 coating appeared to have good self-mating adhesive wear resistance when used in liquid sodium.
In addition to liquid sodium applications, the chromium carbide base thermal spray coating family has been in use for many years to provide sliding and impact wear resistance at elevated temperatures. The most frequently used system by far is the chromium carbide plus nickel chromium composite. The nickel chromium (usually Ni--20 Cr) constituent of the coating has ranged from about 10 to about 35 wt. %. These coatings have been produced using all types of thermal spray processes including plasma spray deposition as well as detonation gun deposition. The powder used for thermal spray deposition is usually a simple mechanical blend of the two components. While the chromium carbide component of the powder is usually Cr.sub.3 C.sub.2, the as-deposited coatings typically contain a preponderance of Cr.sub.7 C.sub.3 along with lesser amounts of Cr.sub.3 C.sub.2 and Cr.sub.23 C.sub.6. The difference between the powder composition and the as-deposited coating is due to the oxidation of the Cr.sub.3 C.sub.2 with consequent loss of carbon. Oxidation may occur in detonation gun deposition as a result of oxygen or carbon dioxide in the detonation gases, while oxidation in plasma spraying occurs as a result of inspiration of air into the plasma stream. Those coatings with a relatively high volume fraction of the metallic component have been used for self-mating wear resistance in gas turbine components at elevated temperatures. These coatings, because of the high metallic content, have good impact as well as fretting wear and oxidation resistance. At lower temperatures, coatings with nominally 20 wt. % nickel-chromium have been used for wear against carbon and carbon graphite in mechanical seals, and for wear in general in adhesive and abrasive applications. These coatings are most frequently produced by thermal spraying. In this family of coating processes, the coating material, usually in the form of powder, is heated to near its melting point, accelerated to a high velocity, and impinged upon the surface to be coated. The particles strike the surface and flow laterally to form thin lenticular particles, frequently called splats, which randomly interleaf and overlap to form the coating. The family of thermal spray coatings includes detonation gun deposition, oxy-fuel flame spraying, high velocity oxy-fuel deposition, and plasma spray.
It is an object of the present invention to provide a process of coating gas path components of turbo machines which comprises thermal spraying chromium carbide and an age hardenable nickel base alloy on the surface of the components.
It is another object of the present invention to provide a process for depositing a coating comprising chromium carbide and an age hardenable nickel base alloy, such as Inconel 718, onto a surface of a turbo machine gas path component and then heat treating the coated surface of the gas path component.
It is another object of the invention to provide an improved erosion resistant coating for gas path components of turbo machines comprising a chromium carbide plus age hardenable nickel base alloy coating.
It is another object of the invention to provide a heat treated thermal spray deposited Cr.sub.3 C.sub.2 +Inconel 718 coating for a gas path component of turbo machines.
The foregoing and additional objects will become more apparent from the description and disclosure hereinafter set forth.